It is said that life time of buildings are determined by such factors as "inadaptability of location" and "functional obsolescence" as well as physical degradation. From the architectural planning point of view, this paper defines "functional obsolescence" of public buildings as caused by inadaptability of buildings to cope with changed regional demand as the years rolled on. Functional conversion is a means solving this kind of inadaptability. Based on the field survey of convented public buildings, the author indicates some tendencies according to different types of buildings for example, goverment offices, public nurseries kindergarten and so forth. She also shows that changing regional demand makes some types of buildings needed or not needed in course of time. Then factors of conversion are examined in accordance wiht the three phases. Firstly, almost all the conversion of puplic buildings takes place after the old facilities moved to newly constructed buildings or they stopped running, leaving the old buildings unoccuppied. Secondly, conversion is made if the vacant buildings are solid enough, but about 40 persent of them are used provisionally untill the definite plans are carried out, or the cases that they cannot be reconstructed for some reasons. Thirdly most of them adopt new functions complying with regional demand or accepting institutions which have been searching for suitable buildings.